The present invention relates generally to fluid ejection devices, and more particularly to a power switching transistor for a fluid ejection device.
One type of conventional fluid ejection system is an inkjet printing system which includes a printhead, a fluid supply which supplies liquid ink to the printhead, and an electronic controller which controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles and toward a print medium, such as a sheet of paper, so as to print onto the print medium. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the printhead and the print medium are moved relative to each other.
Typically, the printhead ejects the ink drops through the nozzles by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as thin film resistors. Heating the ink causes the ink to vaporize and be ejected from the nozzles. Typically, for one dot of ink, a printhead controller controls activation of an electrical current from a power supply. The electrical current is passed through a selected thin film resistor to heat the ink in a corresponding selected vaporization chamber.
In one type of printhead, a power switching device, such as a field effect transistor (FET), is coupled to each thin film resistor to control the application of the electrical current through the thin film resistors. Power is supplied to the thin film resistors via a power supply, which is included as part of the inkjet printing system.
Printer system designs are trending toward the use of printheads with greater numbers of nozzles, more dots per inch, larger swath heights and higher firing frequencies. These features allow for faster printing with higher resolution. Increasing the resistance of the thin film resistors is one way to enable the desired features. This is because the common mode energy variation typically associated with higher nozzle counts can be reduced by using higher resistance thin film resistors. However, this requires the use of higher voltages and power switching devices that can support these voltages. This is because to heat the ink, FETs supply power to the thin film resistors where the power consumed by the thin film resistors is equal to V2/R, where V is the operating voltage supplied to the FET and R is the value of the thin film resistor. Thus, if higher resistance thin film resistors are used, higher operating voltages are required to reach the turn-on energy of a print head.
For reasons stated above and for other reasons presented in the Detailed Description section of the present specification, an approach is desired which will increase the operating voltage level of the FETs used to supply power to thin film resistors in inkjet printheads.
One aspect of the present invention provides a method of manufacturing a power switching transistor for a fluid ejection device. The method includes forming a first conductivity type region. The method includes forming a first diffused region within the first conductivity type region, wherein the first diffused region has a first conductivity type and has a greater impurity concentration than the first conductivity type region. The method includes forming a gate defined to have a thin oxide region and a thick oxide region. The thick oxide region and a first portion of the thin oxide region are disposed over the first conductivity type region. The thin oxide region is at a defined distance from the first diffused region.